A new trial of a microbicide gel to protect women from infection with H.I.V. was canceled Friday after researchers reported that it was not working. The news was a major disappointment for AIDS research.

Finding a vaginal gel that protects women against the virus that causes AIDS but still allows them to get pregnant has long been sought by AIDS researchers, because it can be used secretly by women who fear being refused or even beaten if they ask their sexual partner to use a condom.

In a statement, lead researchers Sharon L. Hillier and Ian McGowan speculated that the problem might have been that too few women used the gel regularly, that the dosing schedule was wrong, or that it somehow caused inflammation that led to easier entry by the virus. But, Dr. Hillier added, it was unlikely that they would be able to assess that until later next year.

In this case, 6 percent of women using the tenofovir gel and 6 percent of those using the placebo had become infected by the time the outside panel looked at the data. It was found to be safe but not effective, which ethically requires the cancellation of the trial to keep any more women from becoming infected.

The trial is expected to go on until mid-2012 and the data are to be released in early 2013. Other trials of gels at different formulations and dosing are planned or under way.